style: Implement the body text color quirk in a more straight-forward way.

This avoids grabbing the document when values that inherit from the body,
whatever that means, aren't under the body.

In that case we'll get a semi-random value, but that's also mishandled by Gecko
anyways (and probably Blink, though haven't tested), and doesn't really make
much sense.

components/style/gecko/generated/bindings.rs

@@ -1551,8 +1551,7 @@ extern "C" {
                                                  *mut nsCSSCounterStyleRule);
 }
 extern "C" {
-    pub fn Gecko_GetBody(pres_context: RawGeckoPresContextBorrowed)
-     -> RawGeckoElementBorrowedOrNull;
+    pub fn Gecko_IsDocumentBody(element: RawGeckoElementBorrowed) -> bool;
 }
 extern "C" {
     pub fn Gecko_GetLookAndFeelSystemColor(color_id: i32,